AMORPHIZATION AND PHASE-TRANSFORMATIONS IN MECHANICALLY ALLOYED TI-AL POWDERS - ELECTRON-MICROSCOPY INVESTIGATION

The microstructural evolution during mechanical alloying of Ti and Al powders has been investigated by X-ray diffraction, scanning electron microscopy (SEM)-energy dispersive spectroscopy, and transmission electron microscopy (TEM). Observations by SEM showed a progressive change of the powders' morphology as a function of milling time. Observations by TEM, performed on a sample milled for 20 h, revealed the simultaneous occurrence of amorphous zones and nanocrystalline domains. The observed amorphous phase is not the final milling product. After 34 h of milling it was possible to identify by TEM fcc (a = 0.41 nm) nanocrystalline zones, with a mean size of about 10 nm. By irradiating the powder milled for 20 h with high density electron beams, a sudden in situ crystallisation took place. The crystallite (fcc with a = 0.41 nm) size was between 0.1 and 0.5 mu-m.